In general, photographic materials for use in photographing the inside of a human body using X-rays include X-ray films for indirectly photographing a visible image produced on a fluorescent screen by X-rays utilizing an optical lens system and X-ray films for direct photography which can record an image formed by direct irradiation with X-rays without utilizing lens system (hereinafter referred to as "direct X-ray filsm").
The present invention is particularly directed to the latter direct X-ray films. Such films usually comprise a transparent support having provided on each side thereof at least one light-sensitive silver halide emulsion layer.
In forming an image on a direct X-ray film by irradiation with X-rays, the film is generally irradiated with X-rays while sandwiched between fluorescent screens, whereby X-ray energy absorbed by the fluorescent screens sandwiching the film is converted to fluorescent light in the blue to green range and the film responds to this fluorescent light to form an image.
Of course, the film responds to the X-ray energy itself to form part of the resulting image, but the proportion of image formed by responding to fluorescent light in the blue to green range overwhelms that formed by responding to X-rays.
As is described above, the combined use of fluorescent screens upon formation of an imag on a direct X-ray film makes it possible to effectively utilize X-ray energy for image formation, and, therefore, provides a great advantage such as reducing the amount of X-ray irradiation upon taking an X-ray photograph of a human body.
However, it is not desirable because sharpness of the image formed is deteriorated.
This defect is based on the following phenomenon. When a direct X-ray film comprising a support having on each side thereof a silver halide emulsion layer is irradiated with X-rays while sandwiched between fluorescent screens, fluorescent light emitted from the fluorescent screen on one side not only forms a latent image (to be developed to a black silver image) in an adjacent silver halide emulsion layer but a considerable portion of the light passes through the support and reaches the silver halide emulsion layer on the opposite side of the support to form another latent image therein, resulting in an indistinct image.
This phenomenon is called "cross-over" in the photographic field.
The degree of this cross-over greatly influences the sharpness of the final image.
An image affected by the cross-over phenomenon appears indistinct seemingly because the fluorescent light diffuses into a silver halide emulsion layer on the opposite side and a support and because diffusion, refraction, and reflection of the light occur around the emulsion layer and the support.
Many studies have been directed to overcome the formation of an indistinct image due to this cross-over phenomenon and to prevent the reduction in sensitivity occurring as a result of reducing the cross-over.
For example, British Pat. No. 1,422,534 discloses a technique of improving sharpness by providing an ultraviolet ray absorbent in a silver halide emulsion layer or between a silver halide emulsion layer and a support to thereby reduce the cross-over.
U.S. Pat. No. 3,989,527 discloses a technique of raising the efficiency of utilizing light and improving sensitivity by incorporating, in an emulsion layer containing spectrally sensitized silver halide grains of 0.9.mu. or above in particle size, silver halide grains free of spectral sensitization and having a particle size of 0.4 to 0.6.mu. as a light-scattering substance. British Pat. No. 504,283 discloses a technique of improving sensitivity by incorporating a pigment such as TiO.sub.2 or ZnO in a silver halide emulsion layer.
In addition, Japanese Patent Application (OPI) Nos. 31737/79 or 69324/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a technique of improving sharpness by incorporating a phosphorescent or fluorescent substance such as CaWO.sub.4 or BaSO.sub.4 in a silver halide emulsion layer or an adjacent layer thereof.
These prior techniques can be roughly classified into the following three types:
(1) reducing cross-over with an ultraviolet ray absorbent, dye, etc.;
(2) effectively utilizing light scattered by a light-scattering substance for raising sensitivity; and
(3) incorporating a luminescent substance in an emulsion layer to thereby raise sharpness and eliminate the necessity of screens.
These prior techniques have the following disadvantages.
The technique of reducing cross-over by absorption (type (1) described above) concurrently causes reduction in sensitivity, thus not being practically employable.
Of the techniques falling into type (2), the technique of incorporating silver halide fine grains as light-scattering substance provides only an insufficient effect with report to improving sharpness though it contributes to increased sensitivity and reduces cross-over. In addition, it does not contribute to photographic characteristics due to low sensitivity of fine grains, and is not preferable in view of the present trend toward saving silver. In contrast, when the grain size is made large in order to improve photographic characteristics, a smaller light-scattering effect is obtained with only a small increase in sensitivity.
When using substances other than silver halide as a light-scattering substance, in many cases, they cannot be removed in development processing and, if removed, they themselves are environmentally problematic. In addition, they provide only an insufficient effect with respect to improving sharpness.
The technique of type (3) is also not practical because the luminescent substance seriously exerts a detrimental influence on photographic properties.
Further, it is known that it is possible to improve sensitivity and gradation of not only X-ray films but light-sensitive materials having two or more silver halide emulsions in general by providing a low sensitive silver halide emulsion layer as a lower layer and a high sensitive silver halide emulsion layer as an upper layer.
However, the conventionally known stratum structure has failed to provide sufficient sensitivity and silver-saving effects.